Magnetic Resonance Imaging (MRI) device has been a practical medical image diagnosing device since 1980s, and at present it is one of the most advanced image diagnosing devices used to discover and diagnose an early-state-cancer and other various diseases. The operating principle of the MRI is shown as follows: a human body is disposed in a particularly-arranged magnetic field; hydrogen atomic nuclei in the human body are excited by radio frequency magnetic field pulses, which makes the hydrogen atoms resonate and absorb energy; radio signals with certain frequencies are emitted from the hydrogen atoms after the radio frequency pulses are stopped, and the energy absorbed is released at the same time; and these signals are recorded by a receiver outside of the body and processed by a computer, to thereby obtain an image.
A main component in MRI is a magnetic device for generating space magnetic field. In order to obtain a clear and real image and accurately diagnose diseases of patients, the magnetic device in MRI is required to generate a homogeneously distributed and sufficiently strong magnetic field in the working area.
MRI can be classified into high field type (having magnetic strength above 3 T in the working area), medium field type (having 1±0.5 T of magnetic strength in the working area), and low field type (having magnetic strength less than 0.4 T in the working area) based on function and image quality. The higher the magnetic strength in the working area, the higher the signal to noise ratio. As a result, a clearer image, which contains more information, can be obtained. The high field type and the medium field type of MRI are fairly advantageous as compared with the low field type of MRI.
Currently, magnets often used for magnetic devices in MRI include superconducting magnets and permanent magnets. The superconducting magnet can generate a strong magnetic field. However, superconducting magnets have complicated structure and it is costly to manufacture. This is why the MRI using the superconducting magnet is very high-priced and operation and maintenance thereof are costly. MRI using the permanent magnet doesn't have these drawbacks, but working magnetic field must be enhanced so that its overall performance approaches that of the MRI using the superconducting magnet.
FIG. 1 is a schematic view of a magnetic device for use in MRI using a permanent magnet in prior art, and FIG. 2a through FIG. 2d show outlines of the permanent magnet used in FIG. 1.
As shown in FIG. 1, the existing magnetic device for use in MRI using a permanent magnet includes a magnetic yoke 1, an upper permanent magnet 2, a lower permanent magnet 3, an upper pole shoe 4, and a lower pole shoe 5. The upper permanent magnet 2 and the lower permanent magnet 3 are attached to the upper end and the lower end of the magnetic yoke 1, respectively, and face each other. The upper pole shoe 4 and the lower pole shoe 5 are disposed on the end surfaces of the upper permanent magnet 2 and the lower permanent magnet 3, respectively. There is an air gap between the upper pole shoe 4 and the lower pole shoe 5. In FIG. 1, an arrow in the upper permanent magnet 2 (or the lower permanent magnet 3) indicates magnetization direction in the magnet, and an arrow in the air gap 6 indicates magnetic field direction generated in the working area.
As shown in 2a through 2d, in the magnetic device for use in MRI in the prior art, the permanent magnet 2 consists of a permanent magnetic material with the same magnetization direction, and has an integrated cylindrical shape, such as cylinder, hollow cylinder, polygonal prism, and hollow polygonal prism. The magnetization direction of the permanent magnetic material is parallel to an axis of the cylinder, as shown by the arrow in the respective magnet.
The magnetic device for use in MRI shown in FIG. 1 has an open C-shape structure. In addition, there are some other structures of magnetic device for use in MRI in the prior art, such as four-column structure, two-column structure, and so on. These magnetic devices are described in detail in IEEE TRANSACTIONS OF APPLIED SUPERCONDUCTIVITY, VOL. 14, NO. 2, JUNE 2004, and Section 1.3.1 of Chapter 1 in MAGNETIC RESONANCE IMAGING, written by M. T. Vlaardingerbroek and J. A. den Boer, the 2nd edition.
The magnetic strength in the working area in the magnetic device for use in MRI in the prior art is only 0.4 T or less, thus, this MRI belongs to low field type. Medium field type of MRI and high field type of MRI generally require superconducting magnets. Therefore, the MRI using a permanent magnet is cheaper and has a good opening, and its structure is relatively simple. However, the magnetic strength generated by the permanent magnet is low. If a larger magnetic strength is required in the working area, an extremely large amount of permanent magnetic material should be used. As a result, the permanent magnet and the MRI including the permanent magnet are very heavy. For example, if 1±0.5 T of magnetic strength (standard strength for medium field type MRI) is required in the working area, 6-12 tons of permanent magnets need to be used. Therefore, there have not been any medium field types of MRIs using a permanent magnet in the prior art. The signal to noise ratio is low and high-speed pulse sequences cannot be performed because the magnetic strength in the conventional MRI using a permanent magnet is low. Accordingly, image definition is lower, and kinds and amount of information obtained are less, as compared to superconducting MRI.
Additionally, permanent magnets and magnetic devices for use in MRI are disclosed in many patent documents. For example, CN1116311A discloses a magnetic field generating apparatus for use in MRI; CN240413Y discloses a magnet apparatus used in magnetic resonance imaging system; CN2430698Y discloses a C shape magnetic resonance imaging permanent magnet without blocking magnetic pole; CN1371000A discloses a completely opened magnetic resonant imaging instrument; CN1400473A discloses a permanent magnet device for magnetic resonance imaging system; CN2542225Y discloses a two-column open C-type permanent-magnet magnetic resonance magnet; CN1491613A discloses a method for assembling magnetic parts for producing magnetic field to form magnetic resonance imaging; and CN1588582A discloses a main magnet body of thin sheet type magnetic field in a fully open magnetic resonance imaging instrument.
However, like the MRI shown in FIG. 1, the permanent magnets or MRI devices disclosed in the above-mentioned documents also have the following drawbacks: because the magnetization strength in the working area is very low (it is only 0.4 T or less), it cannot be used in the medium field type of MRI or the high field type of MRI. Therefore, it is unknown in the art how magnetization strength of a permanent magnet can be enhanced in order to achieve the permanent magnet for use in MRI with higher magnetization strength in the working area, provided that opening degree, consumed amount of the magnetic material, and main sizes and total weight of the magnet all remain unchanged.